Electrochemistry of Polymer-Metal Complexes Citation Neilson, Paul Vincent (1980) Electrochemistry of Polymer-Metal Complexes. Master's thesis, California Institute of Technology. doi:10.7907/HS95-4691. https://resolver.caltech.edu/CaltechTHESIS:04052010-111723098 Abstract Polymeric ligands, Polyethylenimine (PEI) and Chitosan, were studied for possible applications in the modification of electrode surfaces in relation to the three main goals: 1. metal ion scavenging 2. to find a system which catalyzes O 2 reduction 3. understanding of electron transfer in polymer-metal films. Item Type: Thesis (Master's thesis) Subject Keywords: Chemistry Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Anson, Fred C. Thesis Committee: Unknown, Unknown Defense Date: September 1979 Record Number: CaltechTHESIS:04052010-111723098 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:04052010-111723098 DOI: 10.7907/HS95-4691 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 5689 Collection: CaltechTHESIS Deposited By: Tony Diaz Deposited On: 19 Apr 2010 16:59 Last Modified: 31 Jul 2020 20:51 Thesis Files Preview PDF - Final Version See Usage Policy. 823kB Repository Staff Only: item control page

Electrochemistry of Polymer-metal Complexes Paul V. Neilson Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science California Institute of Technology 1980 (Submitted September 1979) To my Parents and Friends Acknowledgement I thank Fred Anson, the members of his group, and the Caltech community for many insightful discussions. ii Abstract Polymeric ligands, Polyethylenimine (PEI) and Chitosan, were studied for possible applications in the modification of electrode surfaces in relation to the three main goals: J. metal ion scavenging 2. to find a system which catalyzes 0, reduction 3. understanding of electron transfer in polymer-metal films. Table of Contents Page Acknowledaement j Abstract i Introduction ] Discussion Chitosan 3 Polyethyleneimine (PEI) 5 Experimental 29 References 31 Introduction The modification of electrode surfaces has been an area very actively researched lately. The advances which have occured since January 1978 include modification of carbon surfaces ,!79 semiconductor 10-17 18-20 21,22 surfaces, metal surfaces, others and a review has appeared.°> The field is past the early problems of getting anything to attach. More advanced studies such as electron transfer rates are now being investigated, -¢/ 28-42 complemented by surface and inter- face studies. The types of molecules attached have also been varied considerably. 1,6,19 12,13 11,18,25,26,43,44 an Porohyrin systems, ferrocenes d 17 dyes, chlorophy]] ° are but a few of the systems which have been attached to electrodes. A recent branch of modified electrodes, polymer coated electrodes, is undergoing the same history. At first, reports dealt with the attachment scheme itself. Now they deal with efforts such as applying these systems to catalysis’? °° and even polymeric electrode materials have appeared. ©! 63 These efforts have been paralleled by work on the characterization and catalytic activity of polymer metal systems. Systems such as Polyvinylpyridine 7 °2 polymethacrylic acid, 0772 polyacrylic acid, 294777 polybipyridine,/0780 and others®! 785 with various metals have been studied. It is hoped from these studies to prepare a polymer metal system in which the catalytic site is well defined, stable, and able to catalyze a useful reaction such as 05 reduction.©°" 20 Nature is able to catalyze 5 reduction effectively by enzymes which contain multiple metal centers in well defined arrangements which are stable. Polymers modified with binuclear or cluster metal complexes may be able to mimic this behavior (cf. polymer-heme 91592) complexes Polymer-single metal complexes are a good starting point for study. Electron transfer studies will complement the 05 reduction work in that structures are needed in which the electron transfer rate from the electrode to all parts of polymer-metal complex are optimized for high current density. 93 ,94 Cyclic voltammetry and chronocoulometry will be the techniques of choice. Chitosan Chitosan is known for binding many metals and is under consideration for the removal of metal ions from waste water by the United States Department of Agriculture. 9797 It is a polyglucosamine made by N-deacetylation of chitin. Its structure, as compared with other known biopolymers, is given below: CH20H CH20H Oo re) OH 0 OH O Cellulose OH OH CH2OH . CH20H ° Q Chit itin 0 OH 0 OH 0 NHCOCH3 NHCOCH3 H CH.OH Chitoson 0 OH 0 OH O° NHe NH2 Chitosan is insoluble in water and a variety of other solvents, but is soluble in aqueous acid. Solutions of .05% or .1% chitosan in .1 M trifluoroacetic acid (HTFA) were used in the following experiments. Basal plane pyrolytic graphite (BPG) electrodes were dipped in the acid-chitosan solution, rinsed with water, and then dipped into Feet or Fe°t (1mM as ammonium sulfates). After rinsing, the electrode was placed in a cell containing only Supporting electrolyte (1M NaTFA (pH ~5.7) or .1M HTFA) where the differential pulse and cyclic voltammograms were recorded. No firm evidence that there was iron or chitosan on the surface was observed. The main problem was that the electrochemistry of the iron complex used was very irreversible, as shown by 200 mV/sec cyclic voltammograms of the solutions below: Anodic Cathodic Solution Peak Potential Peak Potential 2.17 mM Fe°* 0.82 0.18 1% chitosan, 2.21 mM Fe?" 0.91 -0.09 The supporting electrolyte was .1 M HTFA. Even after dipping the chitosan dipped electrode into NaOH solution, in the hope of forming insoluble chitosan on the surface, and then dipping it into Fe?* solution, the electrode showed no iron waves in 2+ iron free supporting electrolyte. Pb was also tried unsuccessfully. Since one of the goals of this research is to find a metal-polymer complex which is easy to attach and maintain on an electrode, other « systems were examined (vide infra). Polyethylenimine (PEI) PEI is a good choice for possible attachment to graphite for a number of reasons. PEI can be chemically modified to produce polymer systems with many different characteristics for use as flocculants, adhesives, ion exchangers, and chelates. PEI and its derivatives form complexes with Cu, Hg, Ni, Cr, Co, Ag, Cd, Pb, Fe, and Zn. PO The ease of modification of the polymer is very important. The metal-PEI complex may have to be "fine turned" (redox potential and other properties) to be an effective catalyst. PEI is a very versatile polymer. Properties of PEI PEI is made by the following reaction: °° —CH,CH2-NH, 25% N Acid 7 1 ‘N-H ————» (CoHsN), 2 -CH2CH2-NH 50% H.C* Catalyst . ‘ 7 -CH.2CH, “N 25% As one can see it is highly branched and can be depicted as % S /- joao >_> ~ oe ? X The PEI worked with had average molecular weights of 600 and 40,000. PEI (600) is an anhydrous liquid which is soluble in water (in all proportions) and other solvents such as DMF and methanol. The micro- analysis is given below: Calculated PEI(600) 55.8 32.5 11.7 100 Sample 1] 50.5 31.5 11.2 93.2 Sample 2 52.4 32.2 11.4 96.0 Sample 1 and samole 2 are from the same bottle but were obtained about two months apart (sample 2 after sample 1). Whereas sample 1's bottle has been opened and closed many times, sample 2's has been opened only a few times. The discrepancy between them is almost certainly due to water absorption. Titration of PEI(600) was also done. The titration in water can be seen in Figure 1. As one can see, PEI is a weak polybase. There- fore, the titration was performed using glacial acetic acid as solvent with HC10, in glacial acetic acid as titrant according to established procedure, !02 The nonaqueous titration is shown in Figure 2. The nonaqueous titration end-point is obviously good only to about 5% (probably too much water in polymer). m1 .Q981M HTFA added pH titration of .0342g PEI(600) in 10.00 mj HA 9 Figure ] 4OO+- ; O60 SF 10 I§ 20 m) .1029M HC10q (in glacial acetic acid) added oh e + = -~ Potentiometric titration of 0.0611 g PEI(600) in 150 ml alacial acetic acid Figure 2 4 Titration Analysis - (PEI-600) g 0 equivatent 61.3 4 5% or “a 16.3 The Jab analysis may be reexpressed as: Lab Analysis (PEI-600) Calculated Sample ] Sample 2 9 43.1 44.5 43.5 equiv. > 23.2 22.5 23.0 In other words, only about 70% of the nitrogens are titratable. This is comparable to titration data in the literature and is probably, in part, attributable to the high charge density that PEI 99,105,106 attains upon protonation. (cf. Lab analysis 23 = - most resins only have about 4 meq/a). PEI and chitosan are cationic 106,107 polyelectrolytes and so are their metal complexes. Poly- electrolyte theory will therefore help in understanding their 108-112 properties (vide infra). 10 For comparison, PEI-40k's lab analysis is shown below: PET-40k 30.5 17.7 10.8 Calculated 27.9 16,25 11.4 PET-40k is a 50% aqueous solution. i, Cu - PEI Complexes The visible spectrum of cue* has one band at 630 nm with an extinction coefficient of 163 M7! cm™!, Nonogaki et al!!3 found a band at 650 nm with an extinction coefficient of 180 mM"! cm7/. However, experimental conditions were not clearly specified making comparison difficult. cu2* or PEI does not have a band in this region, making spectroelectrochemical measurements a possibility, although PEI-cue* may have too low an extinction coefficient for the quantity on the electrode to be determined. ot Electrochemistry, Cu” - PEI(600) 2 Cyclic voltammetry (CV) of the attached Cu * _PEI(600) is shown in Figure 3. CV shows ca. 5 uC of complex attached. The prominent aan Ze ) that one observes for cue* anodic stripping wave (Cu > Cu solutions at .] - .2 V (ca. 100 times as large as the waves of Figure 3), is not present in the cue*-pET CV on the BPG electrode. It is observed with 2.0 m™ CuSO, + 5H,0/14 mM en, however. cu2* en waves on BPG and HMDE are twice as big as cut per waves (same concentration of cue* and nitrogen) leading one to suspect that attachment to the polymer slows down the diffusion rate. The electrochemistry of cue*en on HMDE is a quasi-reversible 1 electron process (60 mV peak separation at low scan rate - increases with scan rate). Selected Ey/2'8 are shown in Table I. cue" PET, however, shows widely separated anodic (plateau) and cathodic peak potentials as shown in Figure 4. 12 Volts vs SSCE weee Cyclic voltammogram of a BPG electrode which has been dipped in a solution containing 2.15 mM PEI(600), 2.02 mM CuS0,-_5H20 and 1M NaTFA for 15 minutes (ca. 14 Nitrogens per Cu2t), The electrode was then rinsed with water and transferred to a solution containing only 1M NaTFA {pH = 7.9, rises to 8.4 at end) where the electrochemistry was done. wwe Background, IM NaTFA only Scan rates: outer curves, 500 mV/sec inner curves, 200 mV/sec Figure 3 13 Table ] Selected potential vs SCE Reaction E vs. SCE Cut + em = Cu 271 cue* + 2e7 = Cu 093 - + cut +e = Cu -.097 Cu(en),“* + e= Cu(en), -.63 ( concentrations not specified) - + Cu(en) 2*+ e = Cu(en), -.54 2 (1.07 M en, 23 mM Cut) Ref 114 114 114 114,115 116 14 ¥ AOnA +——— } 1 ae “t 76 Volts vs SSCE On-n Cyclic voltammoaram of a solution containina: 2.92 mm CuS0,°5H,9 2.0 mM PEI(600) 1 M NaTFA A hanaina mercury drop electrode was used pH ~- 10.6 500 mV/sec Figure 4 15 Figure 5 shows a CV of the dipping solution of Figure 3. In spite of the fact that both Figures 3 and 5 have contributions due to adsorption and Figure 5 should have a contribution due to diffusion, also, both have about the same peak currents. The peaks of Fiqure 5 decrease slightly with cyclying. Figures 4 and 5 further support the hypothesis that the polymer coating is slowing down the diffusion rate. Controlled potential electrolysis (CPE) was attempted using graphite cloth electrode since a large surface area electrode was required and mercury electrodes did not show the same behavior as graphite. The backaround current on araphite cloth was so large that no useful information was obtained. Figure 6 shows the surface coverage, calculated by integration of the cyclic voltammograms, vs time behavior of the electrode of Figure 3. The PEI sample used has a small molecular weight (600), so it is not surprising that it falls off of the electrode rather rapidly, |!7 Not shown in Figure 6 is the fact that this coverace {s potential dependent. If a dipped electrode is held at +0.8 V, there will be no electrochemical response from this electrode, although an electrode held at +.5 V or -1.0 V for 30 seconds shows no significant loss of activity. Some degradation of the polymer at +0.8 V may have occurred. A 2.01 mM PEI (1 M NaTFA supporting electrolyte) solutions Shows oxidation current at about +.6 V which supports this interpretation. The polymer-metal complex may also be put on electrodes by dipping first into polymer solution and then into copper solution. However, Té 20yh x '% ta o o e* ae” ° ae o& aoae 0) eas ues A @ =o © e208 82 @ © @ » P) errreerae ence? Ce ed 5.30. i713 79 77 79 Cyclic voltammoaram of a solution containina: 2.02 mM CuS9,*5H,9 1.97 mM PET(600) J] M NaTFA BPG electrode ammme 500 m\/sec eee 290 mV/sec Figure 5 7 10 o 0% 8 x10 | + "88 O 4 { + QO 20 40 60 80 Time (minutes) Surface coverage vs. time for the electrode of Figure 5. @ cathodic scan O anodic scan The electrode is on open circuit between scans. Figure 6 18 this procedure shows the importance of the cue* /PEI ratio. Figure 7 shows the effect of dipping a PEI coated electrode into a concentrated solution of CuS0,°5H,0. This behavior may be readily understood by examining Figure 8. An electrode dipped into base (NaNH) and then into a cue* solution (Figure 8) shows the same waves as Figure 7. If variable concentrations of dipping PEI solutions are tried, broad waves centered around -.49 V show up which last as long as Figure 3's waves. This may be due to a variable number of ligands bound to cue 118 If pH 1 supporting electrolyte is used no electrochemistry will be observed. The complex is destroyed by strong acid as expected from 119-124 complex stability constant data in the literature for PEI, ammonia, ethylenediamine or tetren. 19 Volts vs. SSCE Cyclic Voltammograms of BPG electrode which has been soaked 15 minutes in a solution containing 2.01 mM PEI (600) (and 1M NaTFA), then soaked in a solution containina -IM CuSO4°5H50 (pH = 4.27). The electrode was then rinsed and transferred to a IM NaTFA supporting electrolyte where electrochemistry was done. Scan rates: m= 500 mV/sec wcoaee 200 mV/sec Figure 7 O 20) OA ke —_ | —$_ = IO. i | | | i } t | r j rn | P64 2024 Volts vs. SSCE Cyclic voltammograms of BPG electrode which has been soaked for 15 minutes in .1M NaOH and then for 15 minutes in Cuc* solution and then transferred to 1M NaTFA (pH = 7.7) supporting electrolyte where electrochemistry was performed. Scan rates: Quter curves 500 mV/sec Inner curves 200 mV/sec The electrode was rinsed with water and blotted dry between each step. Figure 8 2] Electrochemistry, Cu-*-PEI (40k) 2 Since Cu *_PEI (600) fell off the electrode rapidly (cf. Fiaure 8), PEI of higher molecular weight was used. Figure 9 shows a CV ona BPG electrode dipped into PEI(40k) and then into cue, The symmetric wave is very similar to the waves produced when the same procedure is used with PEI(600). The Cu waves produced for the PEI(40k) are longer lived than the waves for PEI(600). More electroactive Cu is found on the electrode surface with the higher MW polymer although the amount is still not reproducible, It is for the above reasons that PEI(40k) was used in the 05 and H0, reduction work. 0» and Hy0, Reduction - cu2*-PEI (49k) 2 Figure 10 shows that Cu * PET (40k) does indeed catalyze 9 2 reduction by about 290 mV (Figure 10 only shows first scans - subsequent scans are smaller). An electrode dipped in a solution of 1 M NaTFA/ 10 m4 borate/5 m™ cut also shows a 200 mV shift, however. This may be due to some kind of Cu-hydroxide species on the electrode surface. Whatever it is falls off after the catalysis experiment, whereas not as much Cu-*-PEI(40k) falls off. 2 Figure 11 shows that Cu * PEI (40k) also catalyzes Ho0. reduction. cu2* per (40k) also falls off after this experiment. Haas et ays find that peroxide oxidizes PEI which is accompanied by some chain degradation. Since the intermediate of the reduction of 0, is presumably a peroxide species, both fall offs may be rationalized as peroxide decomposition of the polymer. This obviously makes cue* -pET unattractive as a 05 reduction catalyst. See for example 125-139. 22 Volts vs SSE ememme (CV of a BPG electrode which has been soaked for 18 minutes in a solution of .6533 a PEI(49k)/25 ml H,9 and then soaked for 15 minutes in a solution of 2.61 m CuSO, -5H,9/ 1 ™“ NaTFA and then transferred to a solution contaifina 1 M NaTFA/10 m™M borate. @e@e@edioned in PEI only. inner curves: 200 mV/sec uter curves: 500 mV/sec Figure 9 23 . S ° 3 . | ° | . 3 . ‘ ° Volts vs SCE CV (first scan) of 9, saturated solution with the BPG electrodes of Figure 9, 2+ emmme Cul /PEI (40k) @we@e@e PEI(40k) only upper curves 500 mV/sec Tower curves 200 mV/sec (200 mV/sec solid curve not shown from +,3 - -.3 due to overlan with dashed curves) Figure 10 24 t—{—_+ ‘§ .30.) 7] 73 75 77 Volts vs SCE +— | BPG electrode soaked for 27 minutes in PEI(49k) and then for 15 minutes in Cu’ (see Fiaure 9). PEI(40k) only Solution: 1 M NaTFA/10 mM borate 20 mM Hp95 pH = 8.1 Fiqure 13 25 2 Summary of Cu * PEI PET was found to scavenge cue* from solution and the resulting cut -pEI was found to catalyze 0, and H50, reduction, although PEI was unnecessary for catalysis of 5 reduction. Evidence was presented to suggest that PEI slowed down the diffusion rate as comnared with en. 26 Co-PEL The cobalt(II) complex of PEI was also tried using mainly PEI(40k). PEI(40k) was found to scavenge coet from solution. Pickup of co2* from a 1 mM solution of Co** by a PEI(40k) coated electrode is complete after about 20 minutes. Increasing the concentration of Co°t from 1 mM to 5 mM and then to 10 mM has no effect after the initial 20 minutes. Figure 12 shows a result of a PEI(40k) coated electrode in 1 mM Co*t The waves are very stable. There is virtually no decrease after 4 hours. If this electrode is taken out of solution and dried and then retried 2 days later in Co2* free supporting electrolyte, the same waves are observed, although smaller. These waves are also very stable - very little decrease is observed after 6 hours. CV of a solution of Cott with an uncoated electrode is equivalent to background over Figure 12's scan ranges. An electrode which has been dipped into base (NaOH) and then into Coot solution also shows no waves in this region unlike cue* There is evidence of the pclymer "“aqumming up" the electron transfer. Integration of Figure 12's waves shows about 10 ul of charge on the surface whereas chronocoulometry of the same electrode shows about 30 - 50 uC of charge over a blank BPG electrode (which shows 20 - 40 nC) for a 700 sec cathodic step. Figure 13 shows the CV of [co(en),]C1, with excess en on BPG electrode. The same solution shows reversible behavior on HMDE electrode (linear current vs (scan rate) '/2), A 2 ™™M Cot and 2.3 mM PEI(600) (pH = 10.5) shows no waves on BPG. No catalysis of 05 or H495 reduction by Co-PEI(40k) was observed. 27 {|__| |} tout 73 7.8 57 77 Volts vs SCE BPG electrode which has been soaked in a solution containina .6533 a PEI(49k)/25 ml and immersed in 34 solution containing 1 M NaTFA/10 mM borate/1? m Co where CV was then performed. Figure 12 28 td —emeeunn eee OV Volts vs SSCE CV of a solution containina: 1.91 mM [Co(en)]C1, 11 m™ en 1 M NaTFA pH = 13.05 emmmen 500 mV/sec e@ee200 mV/sec Figure 13 29 Experimenta] Reagents Reagent grade chemicals such as CuSO, *5H.0 (Mallinckrodt), perchloric acid (Baker), acetit anhydride (Mallinckrodt), ferric and ferrous ammonium sulfate (Mallinckrodt and Baker respectively), cobaltous sulfate (Baker), and acetic acid (Mallinckrodt) were used as received. PEI(600) (Polyscience) and PEI(40k) (Cordova Chemical - 50% solution) were also used as received. Trifluoracetic acid (MCB) was used as received. NaTFA was prepared by neutralizing HTFA with 50% NaQH solution. Borate buffer was prepared from NaoB ,0_-19H,0. Electrochemistry was performed on solutions which were degassed by passing argon through hot copper turnings (ca. 450° C) and then distilled again before use or passed through a Barnstead deionizer. Apparatus Cyclic and differential pulse voltammetry was performed by a PAR model 174 A polarographic analyzer with a Houston Instrument 2000 x-y recorder. Potentiometric titrations were carried out with a Leeds and Northrup model 7664 pH meter. An Orion model 701 A digital pH meter was used for pH titrations. The usual 3 electrode cell was used. 30 A Cary model 14 UV-visible spectrometer was used for visible spectrum measurements. 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